Structure-activity relationships of met5- and leu5-enkephalin analogues.

1. The effect of various analogues of met5- and leu5-enkephalin were determined on the reduction in twitch height of the electrically-stimulated longitudinal muscle preparation of the guinea-pig ileum and of the isolated mouse vas deferens. 2. In the guinea-pig ileum, D-alanine2-met5-enkephalin was the most potent whereas leu5-enkephalin was the most potent in the mouse vas deferens. 3. The met5-enkephalin analogues were more effective in reducing the twitch height of the ileum than they were in depressing that of the vas deferens preparation. The leu5-enkephalin analogues were more potent in their effects on the mouse vas deferens than they were on the guinea-pig ileum. 4. When a peptide bond is replaced by a glycol bond as in glycol2-3-leu5-enkephalin there is a marked reduction in opiate-like activity. 5. Substitution of a D-alanine residue for the glycine2 residue, as in D-alanine2-met5-enkephalin, increases the duration and potency of opiate-like activity. 6. These results confirm that modification of either met5- or leu5-enkephalin can alter the opiate-like potency of the resulting analogues. It appears that an intact tyrosyl residue of leu5-enkephalin is essential for such activity and that substitution of a D-alanine2 residue for the glycine2 residue confers resistance to enzymatic degradation on the met5-enkephalin peptide. In addition, the glycine2-3 peptide bond is essential for opiate-like activity.